1. Field of the Invention
The present invention relates to the production of a silicon semiconductor device, and more particularly to a process for high pressure oxidation of silicon.
2. Description of the Prior Art
When a silicon semiconductor device is produced, it is necessary to form an oxide film (i.e. SiO.sub.2 layer) on a silicon wafer by applying a thermal oxidation process. In a conventional thermal oxidation process the silicon wafer is heated at a high temperature of from 900.degree. to 1200.degree. C., for many hours, in an oxidizing atmosphere, e.g. steam, at ambient pressure. However, when the oxide film is formed by this conventional process, defects of cambers and/or strains arise in the wafer. Such defects arise easily in the large wafers which are frequently used.
In order to eliminate the above-mentioned defects, therefore, a high pressure oxidation process has been proposed. In the high pressure oxidation process the oxidation of a silicon wafer is carried out in a high pressure oxidizing medium, e.g. steam, at a relatively low temperature of from 600.degree. to 1000.degree. C. The features of the high pressure oxidation process are that the oxidizing rate is very rapid, the oxidation of silicon is carried out at a relatively low temperature and the quality of the formed oxide film is better than that of the conventional thermal oxidation process.
In order to carry out the high pressure oxidation process, various methods and apparatuses have been proposed. It was previously known to carry out the high pressure oxidation of silicon in a sealed bomb at a temperature of from 500.degree. to 850.degree. C. and a pressure of from 25 to 500 atm, as mentioned in the reports "Oxidation of Silicon by High-Pressure Steam", by Joseph R. Ligenza, J. Electrochemical Soc., vol. 109, no. 2, pp. 73-76, 1962, and "Selective Oxidation of Silicon in Low-Temperature High-Pressure Steam", by R. J. Powell et al, IEEE Trans. Electron Devices, vol. ED-21, pp. 636-640, October 1974. In this case the bomb is made of Inconel X and the interior thereof is lined with gold. One silicon wafer and a calibrated amount of water are placed inside the gold liner, which is compressed and sealed at the edges by the bomb with bolt joints. The bomb is placed in a furnace (500.degree. to 850.degree. C.) and steam pressure ( 25 to 500 atm) is generated from the water. However, this method has a very serious limitation, in that it is difficult for the bomb to accommodate a large number of silicon wafers.
One experiment with high pressure oxidation schemes was disclosed in the report "Low Temperature Thermal Oxidation of Silicon by Dry Oxygen Pressure above 1 atm", by R. J. Zeto, C. G. Thornton, E. Hryckowian, and C. D. Bosco, J. Electrochem. Soc., vol. 122, no. 10, October 1975, pp. 1409-1410. However, the pressure of the dry oxygen was so high that it would be difficult to construct an industrial apparatus utilizing such pressure.
A high pressure oxidation system is also known from a report of Bell Telephone Laboratories Inc., entitled "High Pressure Steam Apparatus for the Accelerated Oxidation of Silicon", by P. T. Panousis and M. Schneider, Abstract No. 53, of E.C.S. Spring Meeting (E.C.S., New Jersey, 1973), pp. 137-139. The system is of the continuous flow type and uses an adjustable pressure regulator to set the pressure. Operationally, steam is generated in a stainless steel water boiler and piped to a large stainless steel pressure chamber which contains an internal furnace. Silicon wafers are held in a quartz cylinder which is arranged in the furnace. This system is operated at a temperature up to 1200.degree. C. and at a pressure up to 10 atoms. However, the steam comes into contact with the metal of the boiler and chamber, so that there is the possibility of contamination of the wafer with metal ions from the metal. Furthermore, the steam flow influences the uniformity of the thickness of an oxide film grown on the wafers. If the wafers are arranged, standing on edge, at intervals of 2 mm, the uniformity is from .+-.20 to .+-.30%. Therefore, it is necessary to arrange the wafers at intervals of from 10 to 20 mm and, thus, it is difficult to carry out the oxidation of a large number of silicon wafers at one time.
Another known high pressure oxidation system is referred to as the "high pressure pyrogenic oxidation system". This system is disclosed in the report entitled "Oxidation of Silicon in High Pressure Steam", by M. Tsubouchi, H. Miyoshi, A. Nishimoto and H. Abe, Japan Journal of Applied Physics, Vol. 16 (1977), No. 5, pp. 855-856. This system comprises mainly an outer stainless steel chamber, a internal furnace set into the inside of the chamber, and a quartz tube set in the furnace and holding silicon wafers therein. Two gas-feeders of H.sub.2 +O.sub.2 and N.sub.2, which generate steam by H.sub.2 /O.sub.2 reaction, are connected to the inlet of the quartz tube. This system is operated at a temperature of up to 1200.degree. C. and a pressure of up to 9 kg/cm.sup.2. In this system, in order to avoid contamination of the wafers with impurities coming from the chamber and furnace, the inside pressure of the quartz tube is always controlled to be higher than the pressure in the outer stainless steel pressure chamber. However, the construction of the system is complex, and the control of the inside and outside pressure of the quartz tube and of the flow quantity of H.sub.2, O.sub.2 and N.sub.2 is complicated. Furthermore, since there is steam flowing in the quartz tube, the uniformity of the thickness of a grown oxide film and the distance between the wafers are similar to those in the above-mentioned flow type system.